Against the background of a continuously growing volume of data, optical transmission systems are being increasingly used, as they provide very high data rates compared to the bandwidth of electric cables. In optical transmission systems, light waves modulated with the data to be transmitted are generated, coupled into an optical waveguide generally consisting of glass or polymers, and transmitted to the receiver. In practice, wireline optical transmission systems often lack a desirable degree of flexibility.
In recent years, wireless optical transmission systems commonly known as “free-space optical transmission systems” are being increasingly considered for practical applications. In free-space optical transmission systems, the modulated light waves are not coupled into an optical waveguide, but launched into free space and received by a photodetector.
For optical transmission of digital data (binary data), the data is converted in two steps, a serial code sequence (coded sequence of binary data) first being produced by means of an encoding technique and then modulated onto the light waves using a modulation scheme (amplitude, frequency or phase modulation).
Encoding (“line coding”) of the digital data is performed in practice in two ways: either the individual binary values or else the transitions of binary values are encoded as signal levels. In the first case, a so-called NRZ technique (NRZ=non-return-to-zero-level), for example, is used in which a logical “0” is encoded with a low signal level and a logical one “1” with a high signal level. In the second case, so-called Manchester coding is used, for example, whereby each bit period is split into two halves, a logical “1” being represented by a low signal level at the start and a high signal level at the end, and a logical zero “0” in a correspondingly vice versa manner.
If NRZ-encoded data is converted into an amplitude-modulated optical signal, a logical “0” is transmitted with a low signal level (light of little or no intensity) and a logical “1” with a high signal level (light of high intensity). If longer sections (bit blocks) each with the same logical value occur in an NRZ-encoded bit sequence, e.g. “1111110000000”, which contains two longer bit blocks of the logical values “1” and “0”, the light intensity of the optical signal remains unchanged during these bit blocks. In the case of optical transmission in the visible wavelength range, this may be perceived by the viewer as flutter or flicker if the light wave modulation frequency determined by the bit sequence is less than approximately 70 hertz. This can cause the viewer discomfort and even trigger epileptic fits, as medical studies have shown.